The fast development of healthcare technology increases the need for monitoring respiration via the pressure exerted by the human body on appliances, such as a mattress (during sleep), chair and sofa without wearing a sensor. Some applications require a respiration signal to be both DC and relatively noiseless at high-enough quality that enables detection in real time of the onset of breathing phases, e.g. expiration, and breath holding. Such applications may include, for example, breathing pattern modification using guiding tones, as applied by a device called RESPeRATE for inducing relaxation and treating hypertension. This device is described in U.S. Pat. No. 5,800,337. For such applications the sensor should be thin enough not to cause the user any discomfort; flexible, as both the relevant appliances and the human body are usually deformable and soft; highly sensitive, as the pressure modulation elicited by respiration is rather small, and inexpensive, as the main market is for home use. For these reasons, piezo-based sensors that provide AC signals are not optimal.
Flexible sensors that convert pressure into capacitance seem to be appropriate for this purpose. In general, two conductive surfaces of area A separated by a dielectric of thickness d generate capacitance C that is proportional to A/d. Noda et at. (U.S. Pat. No. 7,641,618) disclose a capacitance-based pad sensor for heart/respiration monitoring in bed, in which the dielectric is flexible, resulting in the increase of capacitance, C from its unloaded value due to the reduction of d under loading. Brunner et al. (U.S. Pat. No. 4,986,136) disclose a sensitive capacitance-based pressure measurement system, with an upper conductive surface that includes deformable projections that contact, via a thin dielectric, a lower flat conductive surface (FIG. 3a). The capacitance of this structure increases in response to applied pressure, as the contact area between the upper and the lower conductive surfaces increases due to the deformation of the projections (FIG. 3b). Alternatively, as shown in FIG. 8 thereof both upper- and lower conductive surfaces contain mutually opposed parallel strip-like tapered projections that are oriented to each other preferably at right angles. Respective strips of projections in the upper and lower surface intersect thereby forming a plurality of capacitive cells. The capacitance of the sensor may therefore be considered as a matrix of parallel rows of series connected capacitive cells. In an initial displacement of the upper and lower surfaces, the tips of the projections are undistorted and define very low areas of mutual contact pressure. As the surfaces are urged toward each other, the tips of the projections become progressively flattened thus creating progressively increasing areas of pressure contact. Additionally, the distance between the two surface decreases, which further increases the capacitance. It is to be noted, however, that because the projections of the two surfaces are mutually offset, as indeed they must be to create a matrix of capacitive cells, only their respective tips contribute to the increasing areas of pressure contact. Therefore, at no stage during use of the sensor is there any ability for the opposing projections of the two surfaces to interlock or otherwise engage.
U.S. Pat. No. 4,437,138 discloses a force sensor comprising capacitor plates formed of metallic cloth bonded to a compressible elastomeric dielectric. The metallic cloth strips are in the form of strips running crosswise on opposite sides of the dielectric to provide a matrix of force sensors. The warp and weft threads of the metallic cloth increase the flexibility of the sensor but the warp and weft threads of one plate do not interlock with or otherwise engage the warp and weft threads of the other plate or affect the capacitance of the sensor, which is determined only by the compression of the intermediate dielectric.